This invention relates to a catalyst for the oxidation of sulfur dioxide to sulfur trioxide.
Sulfuric acid is generally prepared on a commercial scale by the gas phase oxidation of sulfur dioxide to sulfur trioxide followed by absorption of the sulfur trioxide in an aqueous medium. Various catalysts for the oxidation of the sulfur dioxide to sulfur trioxide have been described in the prior art.
For example, U.S. Pat. No. Re. 18,380 discloses the use of non-silicious base exchange contact masses containing vanadium as catalysts for this process. U.S. Pat. No. 1,862,825 describes the preparation of a catalyst by spraying calcined natural diatomaceous earth aggregates with a solution of sodium vanadate and drying, preferably in an atmosphere of sulfur dioxide.
Another sulfuric acid catalyst is disclosed in U.S. Pat. No. 3,448,061 wherein a solution prepared by dissolving vanadium pentoxide in sodium or potassium hydroxide solution is mixed with an alkali metal salt of a strong acid, such as potassium sulfate, and the resulting mixture circulated through a preshaped, porous carrier which is subsequently dried to form a catalyst. The process involved the use of excess alkali under controlled pH so as to minimize deformations and breaking of the catalyst carrier on drying or at the working temperature.
In U.S. Pat. No. Re. 19,282, the addition to a very finely divided carrier of potassium or sodium hydrate, carbonate, sulfate, or nitrate together with a vanadium compound is disclosed.
While various vanadium contact catalysts are presently used in commercial operations, they are subject to several disadvantages in their preparation and their use. For example, it is generally recognized that stoichiometric amounts of alkali metal are required in order to confer the desired activity on the catalyst. However, the use of excess amounts of potassium hydroxide causes vanadium to precipitate, thus requiring the addition of ammonia to solubilize the precipitated metal. The use of compounds such as potassium sulfate, as described in the aforementioned '061 patent, require multiple impregnation steps in order to add sufficient vanadium and alkali to the support. Furthermore, while the prior art catalysts may be active or durable, there is still a need for a sulfuric acid catalyst which is both durable and very active.
Now it has been found in accordance with this invention that sulfuric acid catalysts having high activity and durability can be readily made by a one-step process.